Re: MD26-28 400Hz inverter - not very reliable

[Tim Shoppa]

Peter wrote:
> I've had two of the above fail in under 3 years.

I would guess that the failures are related to adverse voltages
coming in or an adverse load on the output. The electrical
environment of a small plane can actually be much more abusive
than a modern car, because of the wide varieties of load dumps.

> Both units developed an intermittent, vibration-related, fault (no
> 400Hz output) which is curious since they are potted and not a lot
can
> move about.

How is it "vibration-related"? Stops working in some positions or
in vibrating environments? Power semiconductor packaging can fail
intermittently due to things you cannot see, it's (as I understand it)
mostly related to repetitive thermal expansion/contraction.

Tim.

[Michael]

> It states that it is "PMA approved" - what does this mean and what is

> required to get such an approval?

It means that a federal bureaucrat has decided that the design and
manufacturing system meet FAA standards, and thus the part can be sold
as an aircraft component.

Since the FAA standards are a bad joke, it doesn't mean the design and
manufacturing system are actually adequate.

The best (but not legal) solution would be to save the box, and to have
someone with a clue build you a real inverter (it's not hard) and then
seal the box up again.

Michael

[Michael]

Peter wrote:
> I can build one allright. I just wondered what is involved in getting
> such an approval for making something like this.

What's mostly involved is spending a bunch of money. The approval
comes from the MIDO, but they won't even talk to you unless you have a
thick sheaf of documents generated by a DER. In theory, this is
someone who knows what he is doing. In practice, it's a cushy
retirement gig for FAA bureaucrats.

You would like to think that the PMA would assure some level of quality
and reliability, but all it assures is a nice retirement income for an
'engineer' who was never employable in the private sector.

Michael

[John Woodgate]

I read in sci.electronics.design that Michael <usenetreplies@thisoldairp
lane.com> wrote (in m
>) about 'MD26-28 400Hz inverter - not very reliable', on Fri, 18 Feb
2005:
>Peter wrote:
>> I can build one allright. I just wondered what is involved in getting
>> such an approval for making something like this.
>
>What's mostly involved is spending a bunch of money. The approval
>comes from the MIDO, but they won't even talk to you unless you have a
>thick sheaf of documents generated by a DER. In theory, this is
>someone who knows what he is doing. In practice, it's a cushy
>retirement gig for FAA bureaucrats.
>
>You would like to think that the PMA would assure some level of quality
>and reliability, but all it assures is a nice retirement income for an
>'engineer' who was never employable in the private sector.
>
>Michael
>
What are MIDO, DER and PMA?

Mass of Indolent Doddering Oldsters?

Don't EVER Retire?

Paradise for Mumbling Ancients?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

[Joerg]

Hello Peter,

>I've had two of the above fail in under 3 years. They drive the KI229
>inverter.
>
>

It sometimes surprises me how often equipment fails even though it is
certified. Like vacuum pumps, for example. There has got to be a way to
improve.

>Both units developed an intermittent, vibration-related, fault (no
>400Hz output) which is curious since they are potted and not a lot can
>move about.
>
>

The only way to find out is if you can simulate the failure on the
ground, preferably in your workshop, and then immediately probe the
components. In this case most likely the transistors. Toroids can also
fail but that would require a stiff mechanical jolt. I have seen some
that looked ok but upon test showed a reduced AL value. Usually hair
cracks.

Regards, Joerg

http://www.analogconsultants.com

[Michael A. Terrell]

John Woodgate wrote:
>
> What are MIDO, DER and PMA?
>
> Mass of Indolent Doddering Oldsters?

Morons, Idiots, Dolts & Oafs


> Don't EVER Retire?

Doddering Elitist Retards!


> Paradise for Mumbling Ancients?

Pathetic & Moronic Accountants?



> Regards, John Woodgate

--
Beware of those who suffer from delusions of adequacy!

Michael A. Terrell
Central Florida

[Rich Grise]

On Sat, 19 Feb 2005 04:08:03 +0000, Michael A. Terrell wrote:

> John Woodgate wrote:
>>
>> What are MIDO, DER and PMA?
>>
>> Mass of Indolent Doddering Oldsters?
>
> Morons, Idiots, Dolts & Oafs
>
>
>> Don't EVER Retire?
>
> Doddering Elitist Retards!
>
>
>> Paradise for Mumbling Ancients?
>
> Pathetic & Moronic Accountants?
>
>
>
>> Regards, John Woodgate

Rather Judicious Warnings?

;-)

[Michael]

John Woodgate wrote:
> What are MIDO, DER and PMA?

Manufacturing Inspection District Office
Designated Engineering Representative
Parts Manufacturing Authorization

Michael

[[email protected]]

On 18 Feb 2005 09:15:12 -0800, "Michael"
> wrote:

>It means that a federal bureaucrat has decided that the design and
>manufacturing system meet FAA standards, and thus the part can be sold
>as an aircraft component.
>
>Since the FAA standards are a bad joke, it doesn't mean the design and
>manufacturing system are actually adequate.
>
>The best (but not legal) solution would be to save the box, and to have
>someone with a clue build you a real inverter (it's not hard) and then
>seal the box up again.
>
>Michael

Had a couple of these inverters powering the AC sync-type fuel flow
indicators in a couple of Beech 18's. The third aircraft had the same
fuel flow lash-up, but was powered by a 6.5 lb motor-verter.

Only had issues with one of the solid state inverters, am thinking it
was an older version of the same unit. I'm assuming the construction
"standard" has changed a little. I could get to the parts inside it,
but they were truly potted in place.

It was by no means a high-tech unit, but the construction at that time
was decent.

One of my all-time favorites is a Honeywell/Grimes 70-0196-1 "solid
state flasher unit". 28v 1.0-5.5 amp input. About the size of a turn
signal/warning light flasher if you potted it into a neat little
anodized aluminum box (hint hint) and ran a couple of wires out the
top. Used to make the red light bulb on the top of a
Chieftain/Cheyenne blink.

Current list price-wait for it-$1035.61, but available at your local
Aviall branch for only $880.27 (order it online, they'll throw in the
shipping).

Mark

[nrp]

Usually solid state stuff should be pretty dependable, but silicon
devices don't like reversed polarity spikes. These can happen if the
master contactor ever stutters when the starter is engaged. I'm
surprised there isn't more transient protection in avionics and solid
state regulators.

I realize that this particular failure sounded like a vibration
problem, but a few more isolating and shunting diodes in our
electronics would seem to help reliability a lot.

[Joerg]

Hello nrp,

>I realize that this particular failure sounded like a vibration
>problem, but a few more isolating and shunting diodes in our
>electronics would seem to help reliability a lot.
>
>

And some ferrites which can be awesome protection for the electronics in
case you get into that surprise thunderstorm.

When debugging EMI in a composite structure back in the 80's I was
mighty disappointed with the EMI susceptibility of some NAV/COM gear.

Regards, Joerg

http://www.analogconsultants.com